1. Field of the Invention
The present invention generally relates to compound semiconductor devices, and more particularly, to a method of manufacturing an optical semiconductor element used for an optical communication and an optical information management.
2. Description of the Related Art
A compound semiconductor has a band structure of a direct transition type and acts with light reciprocally. Therefore, an optical semiconductor device using a compound semiconductor is utilized variously in fields of an optical communication and an optical information management. A semiconductor device of a compound in an InP group, particularly a laser diode, is used for forming an optical signal transferred in an optical fiber with a wavelength in a range of 1.3 to 1.55 mm.
In order to improve a laser oscillation efficiency, it is essential to provide an electric current blocking structure for the laser diode. In the electric current blocking structure, a carrier is shut in a limited area in an axial direction. Besides, it is also essential to shut a light efficiently in the area where the carrier is shut, because the laser oscillation is generated in the laser diode due to a stimulated emission. Shutting the light to a horizontal direction in the laser diode in the InP group is realized on a basis of a difference of a refractive index between InGaAs core wave guiding a light and an InP buried layer.
FIG. 1A to 1D are views for explaining a manufacturing process of a laser diode 10 having a buried hetero structure (BH structure) as an electric current blocking structure and optical confinement structure.
Referring to FIG. 1A, a multi-quantum well layer 12 is formed on an n-type 10P substrate 11. An InGaAs layer and an InGaAs layer are grown repeatedly in the multi-quantum well layer 12. A p-type InP clad layer 13 is formed in the multi-quantum well layer 12, and a p-type InGaAs contact layer 14 layer is formed on the p-type InP clad layer 13.
In a process shown in FIG. 1B following the process shown in FIG. 1A, a SiO2 film 15 is formed on the contact layer 14 as an etching protection layer. After forming of the SiO2 film 15, an active layer mesa stripe is formed by dry etching. The mesa stripe is extended in the <011>-direction in FIG. 1B.
In a process shown in FIG. 1C following the process shown in FIG. 1B, the SiO2 film 15 is used as a selective growth mask. Besides, a crystal growth of an InP buried layers 16A and 16B having high resistances is implemented at both sides of the mesa stripe by a metal organic vapor phase epitaxy (MOVPE) method. The InP buried layers 16A and 16B are formed by Fe doping. During a re-growth process of the InP buried layers 16A and 16B, a growth stop surface (111) B surface is generated, and thereby the InP buried layers 16A and 16B have growth configurations in which mask edges of the InP buried layers 16A and 16B are bulged in areas 16a and 16b. 
In a process shown in FIG. 1D following a process shown in FIG. 1C, the SiO2 film 15 is eliminated. Besides, a p-side electrode 17 is formed on the contact layer 14, and an n-side electrode 18 is formed on a bottom surface of the substrate 11.
Thus, in case of a buried growth of the InP buried layers 16A and 16B when the SiO2 film 15 is used as a selective growth mask, it may be difficult to avoid the bulges of the InP buried layers 16A and 16B in the areas 16a and 16b corresponding to edges of the SiO2 film 15. This is because a material concentration is increased partially on the SiO2 film 15, because the crystal growth is not generated on a mask of the SiO2 film 15. The material is excessively provided on surfaces of the InP buried layers 16A and 16B growing at both sides of the mesa stripe. For instance, during the process shown in FIG. 1C, when the mesa stripe has a height of approximately 1.5 mm, the InP buried layers 16A and 16B are bulged with a height of approximately 0.7 mm in areas 16a and 16b of the mask edges.
As described above, during the process shown in FIG. 1D, the p-side electrode 17 is formed on a surface having a step structure. If the p-side electrode 17 is formed by sputtering a Ti film, a Pt film and an Au film in turn, a disadvantage occurs. That is, because the Ti film and the Pt film have a thickness of approximately 0.1 mm each and the p-side electrode 17 is formed on the surface having the step structure, an electrode layer is broken at a concave-convex part 17a. If the electrode layer is broken, passing of an electric current through such electrode becomes uneven, and thereby an electric deterioration of a device is produced.
Meanwhile, an optical integrated circuit element has received attention recently as an important optical semiconductor device. The laser diode, awave guide, a light-receiving element, and an optical function element are integrated in the optical integrated circuit element. In such an optical integrated circuit element, the mesa stripe may be extended to a direction other than <011> or a branch point may exist in the mesa stripe. If a burying growth of the mesa stripe of the <011>-direction is implemented as shown in FIG. 1C, the growth stop surface (111) B surface is generated. However, regarding a burying growth of the optical integrated circuit element, an overhang part that a buried layer 23 is extended on a SiO2 mask 22 may be generated as shown in FIGS. 3-(A) to 3-(C), because a particular growth stop surface does not exist in the optical integrated circuit clement. Here, FIG. 3-(A) is a perspective view of such the optical wave guide; FIG. 3-(B) is a cross-sectional view thereof; and FIG. 3-(C) is a partially enlarged view thereof.
Referring to FIGS. 3-(A) to 3-(C), a SiO2 pattern 22 having an opening part is formed on an InP substrate 21. The InP substrate 21 is partially exposed by the opening part. An InP buried layer 23 is formed on an partially exposed surface of the InP substrate 21 by using the SiO2 pattern 22 as a mask, through a re-growth process. At this time, because the InP buried layer 23 does not have a growth stop surface such as the growth stop surface (111) B surface, the InP buried layer 23 grows to a side beyond the opening part of the SiO2 pattern 22. As a result of this, an overhang part 23A is formed on the SiO2 pattern 22, as shown in FIG. 3-(C).
As shown in FIG. 4, if an InP layer 24 re-grows after the SiO2 pattern 22 is removed, a material gas does not reach a part directly under the overhang part 23A, and thereby a cave part 23B may be formed thereon. Such a cave part 23B has a quite different refractive index from the InP buried layer 23. Therefore, a light which is wave guided in the wave guide is scattered, and thereby a loss of light is caused.
FIGS. 5A to 5B are views for explaining a problem of a laser diode in the case in which a mesa stripe 31M is extended to a direction other than the <101>-direction, and InP buried layers 32A and 32B are formed on both sides of the mesa stripe 31M by using a SiO2 film 33 formed on the mesa stripe 31M as a selective growth mask.
In an example shown in FIG. 5A, the mesa stripe 31M is extended to an offset direction at an angle of 10° from <011>-direction to <010>-direction. As shown in an enlarged view in FIG. 5A, because the InP buried layers 32A and 32B do not have growth stop surfaces, the InP buried layers 32A and 32B grows onto the SiO2 film 33. As a result of this, the overhang part is generated on the SiO2 film 33.
If the SiO2 mask 33 is removed by etching and an hip layer 34 is grown as covering the InP buried layers 32A and 32B and the mesa stripe 31M, a vapor material does not reach a directly under area of the overhang part, and thereby cave parts 32a and 32b may be formed thereon, as shown in an enlarged view in FIG. 5B.